For example, I don’t use Gmail and I run my own personal Mastodon instance using masto.host, this doesn’t stop me communicating with people on Gmail or major Mastodon instances like mastodon.social.
I mentioned Gmail because, when a single instances holds something like 95% of the users, that gives them a lot of power. If Gmail decided to de-federate from you… you are kinda screwed. That’s my concern. Although, as you said, that is still better than a fully centralized platform.
And that is the thing I have been struggling with and if the major instances looked visually distinct it would make it easier to not confuse them. But yeah, the fediverse has a marketing problem. We need to get people with marketing skills involved.